Methods for forming resistive switching memory elements by heating deposited layers

ABSTRACT

Resistive switching nonvolatile memory elements are provided. A metal-containing layer and an oxide layer for a memory element can be heated using rapid thermal annealing techniques. During heating, the oxide layer may decompose and react with the metal-containing layer. Oxygen from the decomposing oxide layer may form a metal oxide with metal from the metal-containing layer. The resulting metal oxide may exhibit resistive switching for the resistive switching memory elements.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application and claims priority toU.S. application Ser. No. 12/400,655 filed on Mar. 9, 2009. Furthermore,a claim for priority is hereby made under the provisions of 35 U.S.C.§119 for the present application based upon U.S. Provisional ApplicationNo. 61/035,354 entitled “Methods for Forming Resistive Switching MemoryElements by Heating Deposited Layers” and filed on Mar. 10, 2008, whichis incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to nonvolatile memory elements, and moreparticularly, to methods for forming nonvolatile resistive switchingmemory elements.

BACKGROUND OF THE INVENTION

Nonvolatile memory elements are used in systems in which persistentstorage is required. For example, digital cameras use nonvolatile memorycards to store images and digital music players use nonvolatile memoryto store audio data. Nonvolatile memory is also used to persistentlystore data in computer environments.

Nonvolatile memory is often formed using electrically-erasableprogrammable read only memory (EPROM) technology. This type ofnonvolatile memory contains floating gate transistors that can beselectively programmed or erased by application of suitable voltages totheir terminals.

As fabrication techniques improve, it is becoming possible to fabricatenonvolatile memory elements with increasingly small dimensions. However,as device dimensions shrink, scaling issues are posing challenges fortraditional nonvolatile memory technology. This has lead to theinvestigation of alternative nonvolatile memory technologies, includingresistive switching nonvolatile memory.

Resistive switching nonvolatile memory is formed using memory elementsthat have two or more stable states with different resistances. Bistablememory has two stable states. A bistable memory element can be placed ina high resistance state or a low resistance state by application ofvoltage pulses. Nondestructive read operations can be performed toascertain the value of a data bit that is stored in a memory cell.

Nonvolatile memory elements can be formed using metal oxides. Resistiveswitching based on nickel oxide switching elements and noble metalelectrodes such as platinum electrodes has been demonstrated.

In a typical scenario, a stack of resistive switching oxide andelectrode layers is deposited using physical vapor deposition (PVD)(sputtering). Dry etching is then used to pattern the deposited layers.Heat may be applied to the deposited structures during this typefabrication process.

Arrangements such as these may be satisfactory, but can give rise toprocessing challenges. For example, multilayer materials that undergohigh temperature treatments may lose interfacial integrity due tointerdiffusion or stress-induced delamination.

Moreover, the use of conventional rapid thermal oxidation processes orother such post-processing oxidation techniques to form resistiveswitching metal oxide films may lead to undesirable oxidation of contactpads or other device features.

It would therefore be desirable to be able to provide improvedtechniques for forming resistive switching structures using thermalprocesses.

SUMMARY

In accordance with the present invention, nonvolatile memory elementsare formed using in-situ processes. These in-situ processes can be usedto avoid reliance on conventional rapid thermal oxidation techniques andother such post-processing oxidation techniques and may reduce processcomplexity.

The nonvolatile memory elements may contain layers of metal oxide thatexhibit resistive switching. The resistive switching metal oxide layersmay be formed by depositing oxide layers and metal-containing layers andby heating the deposited layers.

With one suitable arrangement, a metal-containing layer such as titaniumnitride may be deposited. An oxide layer such as ruthenium oxide may bedeposited on the metal-containing layer. A rapid thermal annealing toolor other equipment may be used to heat the oxide layer and themetal-containing layer. During heating, oxygen from the oxide layer canoxidize the metal from the metal-containing layer. For example, in ascenario in which a layer of ruthenium oxide that has been deposited ona layer of titanium nitride is heated, the oxygen from the rutheniumoxide layer may oxidize some of the titanium in the titanium nitridelayer, thereby forming a resistive switching layer of titanium oxide.Layers of ruthenium and unoxidized titanium nitride may be formed aboveand below the titanium oxide layer as part of the heating process. Ifdesired, this type of arrangement may be used to oxidize stacks ofmetal-containing layers. For example, an alternating arrangement ofmetal-containing layers and oxide layers may be deposited. Followingheat treatment, each of the metal-containing layers may be oxidized byoxygen from one or more adjoining oxide layers. The metal-containinglayers may be metals, metal alloys, metal nitrides, or any othersuitable metal-containing materials.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings:

FIG. 1 is a diagram of an illustrative array of resistive switchingmemory elements in accordance with an embodiment of the presentinvention.

FIG. 2A is a cross-sectional view of an illustrative resistive switchingnonvolatile memory element in accordance with an embodiment of thepresent invention.

FIG. 2B is a cross-sectional view of an illustrative resistive switchingnonvolatile memory element in accordance with another embodiment of thepresent invention.

FIG. 3 is a graph showing how a resistive switching nonvolatile memoryelement of the types shown in FIGS. 2A and 2B may exhibit bistablebehavior in accordance with an embodiment of the present invention.

FIG. 4 is a schematic diagram of an illustrative resistive switchingmemory element in series with a diode in accordance with an embodimentof the present invention.

FIG. 5 is a schematic diagram of an illustrative resistive switchingmemory element in series with an electrical device in accordance with anembodiment of the present invention.

FIG. 6 is a schematic diagram of an illustrative resistive switchingmemory element in series with two electrical devices in accordance withan embodiment of the present invention.

FIGS. 7, 8, and 9 are cross-sectional side views of an illustrativeresistive switching memory element during fabrication in accordance withan embodiment of the present invention.

FIG. 10 is a cross-sectional side view of another illustrative resistiveswitching memory element during fabrication in accordance with anembodiment of the present invention.

FIG. 11 is a flow chart of illustrative steps involved in fabricating aresistive switching memory element in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

Embodiments of the present invention relate to nonvolatile memory formedfrom resistive switching elements. Embodiments of the invention alsorelate to fabrication methods that may be used to form nonvolatilememory having resistive switching memory elements.

Resistive switching elements may be formed on any suitable type ofintegrated circuit. Most typically, resistive switching memory elementsmay be formed as part of a high-capacity nonvolatile memory integratedcircuit. Nonvolatile memory integrated circuits are often used inportable devices such as digital cameras, mobile telephones, handheldcomputers, and music players. In some arrangements, a nonvolatile memorydevice may be built into mobile equipment such as a cellular telephone.In other arrangements, nonvolatile memory devices are packaged in memorycards or memory keys that can be removably installed in electronicequipment by a user.

The use of resistive switching memory elements to form memory arrays onmemory devices is merely illustrative. In general, any suitableintegrated circuit may be formed using the resistive switchingstructures of the present invention. Fabrication of memory arrays formedof resistive switching memory elements is described herein as anexample.

An illustrative memory array 10 of resistive switching memory elements12 is shown in FIG. 1. Memory array 10 may be part of a memory device orother integrated circuit. Read and write circuitry may be connected tomemory elements 12 using conductors 16 and orthogonal conductors 18.Conductors such as conductors 16 and conductors 18 are sometimesreferred to as word lines and bit lines and are used to read and writedata into the elements 12 of array 10. Individual memory elements 12 orgroups of memory elements 12 can be addressed using appropriate sets ofconductors 16 and 18. Memory element 12 may be formed from one or morelayers of materials, as indicated schematically by lines 14 in FIG. 1.In addition, the memory arrays shown can be stacked in a verticalfashion to make multi-layer 3-D memory arrays.

During a read operation, the state of a memory element 12 can be sensedby applying a sensing voltage to an appropriate set of conductors 16 and18. Depending on its history, a memory element that is addressed in thisway may be in either a high resistance state or a low resistance state.The resistance of the memory element therefore determines what digitaldata is being stored by the memory element. If the memory element has ahigh resistance, for example, the memory element may be said to containa logic one (i.e., a “1” bit). If, on the other hand, the memory elementhas a low resistance, the memory element may be said to contain a logiczero (i.e., a “0” bit). During a write operation, the state of a memoryelement can be changed by application of suitable write signals to anappropriate set of conductors 16 and 18.

A cross-section of an illustrative embodiment of a resistive switchingmemory element is shown in FIG. 2A. In the example of FIG. 2A, memoryelement 12 is formed from a metal oxide 22 and has conductive electrodes20 and 24. When constructed as part of an array such as array 10 of FIG.1, conductive lines such as lines 16 and 18 may be physically andelectrically connected to electrodes 20 and 24. Such conductive linesand electrodes may be formed from any suitable conductive materials suchas metals (e.g., tungsten, aluminum, copper, ruthenium, noble metals,near-noble metals, non-noble metals), metal silicides, metal nitrides(e.g., TiN), metal silicon nitrides, doped polysilicon, doped silicon,combinations of these materials, or any other suitable conductivematerials.

If desired, conductive line 16 and conductive line 18 may serve as bothconductive lines and as electrodes. In this type of arrangement, line 16may serve as electrode 20, so that no separate conductor is needed toform an upper electrode for element 12. Similarly, line 18 may serve aselectrode 24, so that no separate conductor is needed for the lowerelectrode of element 12.

In the diagram of FIG. 2A, conductive lines 16 and 18 are shownschematically as being formed in contact with electrodes 20 and 24.Other arrangements may be used if desired. For example, there may beintervening electrical components (e.g., diodes, p-i-n diodes, silicondiodes, silicon p-i-n diodes, transistors, etc.) that are formed betweenline 16 and electrode 20 or between line 18 and electrode 24.

If desired, there may be an intervening electrical component between anelectrode and resistive switching metal oxide 22. An illustrativearrangement in which there is an intervening electrical component 38between electrode 24 and metal oxide 22 is shown in FIG. 2B.

As indicated schematically by dotted lines 21, conductive materials suchas conductive layers 24 and 20 and resistive switching layers such asmetal oxide layer 22 may be formed from one or more layers of materials.

With one illustrative embodiment, metal oxide 22 may be formed from abinary metal oxide such as titanium oxide. With another illustrativeembodiment, resistive switching may be provided using a layer ofTiO_(x)N_(y) (or MO_(x)N_(y) where M is a metal) as part or all ofresistive switch layer 22. These are merely illustrative examples. Ingeneral, resistive switching layer 22 may be formed using any suitablematerial that exhibits resistive switching.

Resistive switching memory element 12 may exhibit a bistable resistance.When resistive switching memory element 12 is in a high resistancestate, it may be said to contain a logic one. When resistive switchingmemory element 12 is in a low resistance state, it may be said tocontain a logic zero. (If desired, high resistance can signify a logiczero and low resistance can signify a logic one.) The state of resistiveswitching memory element 12 may be sensed by application of a sensingvoltage. When it is desired to change the state of resistive switchingmemory element 12, read and write circuitry may apply suitable controlsignals across terminals 16 and 18.

A current (I) versus voltage (V) plot for device 12 is shown in FIG. 3.Initially, device 12 may be in a high resistance state (e.g., storing alogic one). In this state, the current versus voltage characteristic ofdevice 12 is represented by solid line HRS 26. The high resistance stateof device 12 can be sensed by read and write circuitry 14 (FIG. 1). Forexample, read and write circuitry 14 may apply a read voltage V_(READ)to device 12 and can sense the resulting low current I_(L) that flowsthrough device 12. When it is desired to store a logic zero in device12, device 12 can be placed into its low-resistance state. This may beaccomplished by using read and write circuitry 14 to apply a voltageV_(SET) across terminals 16 and 18 of device 12. Applying V_(SET) todevice 12 causes device 12 to enter its low resistance state, asindicated by dashed line 30. In this region, the structure of device 12is changed (e.g., through the formation of current filaments throughmetal oxide 22 or other suitable mechanisms), so that, following removalof the voltage V_(SET), device 12 is characterized by low resistancecurve LRS 28.

The low resistance state of device 12 can be sensed using read and writecircuitry 14. When a read voltage V_(READ) is applied to resistiveswitching memory element 12, read and write circuitry 14 will sense therelatively high current value I_(H), indicating that device 12 is in itslow resistance state. When it is desired to store a logic one in device12, device can once again be placed in its high resistance state byapplying a voltage V_(RESET) to device 12. When read and write circuitry14 applies V_(RESET) to device 12, device 12 enters its high resistancestate HRS, as indicated by dashed line 32. When the voltage V_(RESET) isremoved from device 12, device 12 will once again be characterized byhigh resistance line HRS 26.

The bistable resistance of resistive switching memory element 12 makesmemory element 12 suitable for storing digital data. Because no changestake place in the stored data in the absence of application of thevoltages V_(SET) and V_(RESET), memory formed from elements such aselement 12 is nonvolatile.

Any suitable read and write circuitry and array layout scheme may beused to construct a nonvolatile memory device from resistive switchingmemory elements such as element 12. For example, horizontal and verticallines 16 and 18 may be connected directly to the terminals of resistiveswitching memory elements 12. This is merely illustrative. If desired,other electrical devices may be associated with each element 12.

An example is shown in FIG. 4. As shown in FIG. 4, a diode 36 may beplaced in series with resistive switching memory element 12. Diode 36may be a Schottky diode, a p-n diode, a p-i-n diode, or any othersuitable diode.

If desired, other electrical components can be formed in series withresistive switching memory element 12. As shown in FIG. 5, electricaldevice 38 may be placed in series with resistive switching memoryelement 12. Device 38 may be a diode, a transistor, or any othersuitable electronic device. Because devices such as these can rectify orotherwise alter current flow, these devices are sometimes referred to asrectifying elements or current steering elements. As shown in FIG. 6,two electrical devices 38 may be placed in series with a resistiveswitching memory element 12.

Memory elements 12 may be formed in a single layer in array 10 or may beformed in multiple layers. An advantage of forming memory arrays such asmemory array 10 of FIG. 1 using a multi-layer memory element scheme isthat this type of approach allows memory element density to bemaximized.

If desired, a resistive switching metal oxide layer may be formed aboveor below a diode (as an example). Conductive lines 16 and 18 may beelectrically coupled to metal oxide 22 through a number of layers ofconductive material. There may, in general, be any suitable number ofconductive layers associated with resistive switching memory element 12.These conductive layers may be used for functions such as adhesionpromotion, seed layers for subsequent electrochemical deposition,diffusion barriers to prevent undesired materials from diffusing intoadjacent structures, contact materials (e.g., metals, metal alloys,metal nitrides, etc.) for forming ohmic contacts with the metal oxide22, contact materials (e.g., metals, metal alloys, metal nitrides, etc.)for forming Schottky contacts to the metal oxide 22, etc.

The conductive layers in element 12 may be formed from the sameconductive material or different conductive materials. For example, theconductive layers of element 12 that contain metal may include the samemetal or different metals. Moreover, conductive layers in element 12 maybe formed using the same techniques or different techniques. As anexample, one layer of metal may be formed using physical vapordeposition (PVD) techniques (e.g., sputter deposition), whereas anotherlayer of metal may be formed using electrochemical deposition.

The portions of the conductive layers in element 12 that are immediatelyadjacent to resistive switching layer 22 or are otherwise in closeassociation with layer 22 are sometimes referred to as the electrodes ofthe resistive switching memory element 12.

In general, the electrodes of resistive switching memory element 12 mayeach include a single material (e.g., a given metal), may each includemultiple materials (e.g., titanium nitride), may include materialsformed using different techniques (e.g., electrochemically depositednickel and PVD nickel), or may include combinations of such materials.

Certain metals are particularly appropriate for forming metal oxide 22.These metals may include, for example, the transition metals and theiralloys. With one particularly suitable arrangement, the metals forforming metal oxide 22 include titanium. The metal oxide 22 may includeother elements in addition to titanium. For example, metal oxide 22 mayinclude titanium, oxygen, and nitrogen. Metals other than titanium mayalso be used for metal oxide 22. Titanium-based metal oxides 22 aresometimes described herein as an example.

Any suitable conductive materials may be used for forming the electrodes20 and 24 of resistive switching memory element 12. Illustrativeconductive materials include transition metals (and their nitrides),refractory metals (and their nitrides), noble metals, and near-noblemetals. Illustrative examples of conductive materials include Ti, Ta, W,Mo, Hf, Nb, Ni, Pd, Pt, Re, Ru, and Ir. Illustrative metal nitridesinclude titanium nitride, tantalum nitride, tungsten nitride, andmolybdenum nitride. These are merely illustrative examples of materialsthat may be used for electrodes 20 and 24. Combinations of two or moreof these materials (and/or their nitrides) may be used or other suitableconductive materials may be used as electrodes 20 and 24, if desired.

The electrodes 20 and 24 and other conductive layers that may beassociated with elements 12 may be formed using any suitable techniques.Illustrative conductive material fabrication techniques include physicalvapor deposition (e.g., sputter deposition, evaporation), chemical vapordeposition, atomic layer deposition, and electrochemical deposition(e.g., electroless deposition, electroplating). Metal oxide 22 may beformed by oxidizing one or more of these conductive materials.

With one particularly suitable arrangement, which is described herein asan example, the oxygen that is used in forming resistive switching metaloxide layer 22 may be provided from a deposited layer of oxide. Thistype of approach is illustrated in connection with FIGS. 7, 8, and 9.

As shown in FIG. 7, a metal-containing layer 40 may be deposited onlayer 42. During subsequent operations, layer 40 may be oxidized byheating layer 40 in the presence of an adjacent oxide layer.

Layer 42 may include one or more layers of material such as a substratematerial (e.g., silicon, glass, etc.), conductors such as conductors 20,24, 16, and 18 of FIGS. 2A and 2B, etc. Metal-containing layer 40 maycontain any metal such as titanium that is suitable for forming aresistive switching layer when oxidized. An example of a suitablematerial for metal-containing layer 40 is titanium nitride (TiN). Anadvantage of a metal-containing layer such as titanium nitride is thatportions of the metal-containing layer of this type may remainunoxidized following heat treatment (i.e., as titanium nitride) and maytherefore be suitable for use in forming all or part of conductors suchas conductors 20, 24, 16, and 18 of FIGS. 2A and 2B. The use of titaniumnitride in metal-containing layer 40 is merely illustrative. Anysuitable materials may be used in metal-containing layer 40. Forexample, metal-containing layer 40 may include multiple metals (i.e., ametal alloy) and the metal or metals of layer 40 may, if desired, becombined with any suitable additional materials (e.g., nitrogen, etc.).Examples of suitable resistive switching metal oxides that may be formedfrom heating layer 40 in the presence of an adjacent oxide layer includeTiO_(x), AlO_(x), HfO_(x), TaO_(x), ZrO_(x), VO_(x), NbO_(x), CrO_(x),MoO_(x), WO_(x), MnO_(x), and MoO_(x). The metal oxides that are formedmay include multiple metals, nitrogen, etc.

Metal-containing layer 40 may be formed on layer 42 using any suitabletechnique such as physical vapor deposition (e.g., sputter deposition,evaporation), chemical vapor deposition, atomic layer deposition, andelectrochemical deposition (e.g., electroless deposition orelectroplating). The thickness of metal-containing layer 40 is typicallyless than a few microns. For example, metal-containing layer 40 may havea thickness of tens, hundreds, or thousands of angstroms (as anexample). Metal-containing layer 40 may contain one or more sublayers ofmaterial. With one illustrative arrangement, which is sometimesdescribed herein as an example, metal-containing layer 40 is formed froma single layer of titanium nitride or other suitable material.

After metal-containing layer 40 has been deposited on layer 42, oxidelayer 44 may be deposited on metal-containing layer 40. This type ofconfiguration is shown in FIG. 8. Oxide layer 44 may include anyoxygen-containing materials that can serve as suitable sources of oxygenfor a resistive switching layer. As an example, oxide layer 44 mayinclude a metal oxide layer such as a binary metal oxide layer. Ifdesired, the metal oxide of layer 44 may be formed from a noble metal ornear-noble metal. For example, layer 44 may be formed from a noble metaloxide such as ruthenium oxide (RuO_(x)). Another example of a suitablemetal oxide material is iridium oxide (IrO_(x)).

Any suitable technique may be used for forming oxide layer 44 onmetal-containing layer 40. Examples of such techniques include physicalvapor deposition (e.g., sputter deposition, evaporation), chemical vapordeposition, atomic layer deposition, and electrochemical deposition(e.g., electroless deposition or electroplating). Oxide layer 44 may beformed to any suitable thickness (e.g., microns, hundreds or thousandsof angstroms, etc.).

After the multilayer structure of FIG. 8 has been formed, heat may beapplied. Any suitable technique may be used for applying heat to themultilayer structures of FIG. 8. As an example, the heat may be appliedby placing the structures in a furnace. If desired, heat may be appliedusing a rapid thermal annealing (RTA) tool. In a typical rapid thermalanneal (RTA) process, the structures may be heated to a temperature of750° C. for about one minute (as an example). Heat treatment may beperformed in the presence of an annealing ambient (e.g., argon) or anyother suitable atmosphere. Heat may be applied for any suitable lengthof time (e.g., more than 10 seconds, less than one minute, more than oneminute, etc.). The maximum temperature to which the structures areraised may be greater than 400° C., greater than 600° C., less than 750°C., may be equal to 750° C., or may be more than 750° C.

When heat is applied to the structures of FIG. 8, oxide layer 44 reactswith layer 40. For example, consider a scenario in which oxide layer 44is formed from ruthenium oxide and in which metal-containing layer 44 isformed from titanium nitride. In this type of situation, heat treatmentcan cause the ruthenium oxide to react with the titanium nitride layer.As these layers react with each other, the oxygen from the rutheniumoxide layer (i.e., layer 44) can react with the titanium from thetitanium nitride layer (i.e., layer 40) to form a resistive switchingmetal oxide layer 48 of titanium oxide, as shown in FIG. 9. It isunderstood that resistive switching, according to various mechanisms,may occur in regions of the memory element other than the resistiveswitching metal oxide layer 48. Because the reaction process depletesoxygen from metal oxide layer 44, a layer of metal such as metal layer46 of FIG. 9 may remain on top of resistive switching layer 48 followingheat treatment. In particular, a layer of ruthenium may remain on top ofthe titanium oxide layer 48 that is formed from the reaction of theoxygen from ruthenium oxide layer 44 and the titanium from titaniumnitride layer 40. In the event that some of the titanium nitride layer40 does not react with the oxygen, a portion of unoxidizedmetal-containing layer 40 may remain on top of additional layer 42, asshown in FIG. 9. It is not necessary for the oxidation reaction toconsume all of the oxygen in layer 44. For example, a metal oxide inoxide layer 44 such as ruthenium oxide may only partially disassociateduring heating so that a layer 46 is produced that is made up of amixture of ruthenium and ruthenium oxide. The partial disassociation mayresult in the oxide layer 44 forming a bilayer 46 having a layer ofruthenium adjacent to the resistive switching layer 48 and a layer ofruthenium further from the resistive switching layer 48. The two layersof ruthenium may be separated by a residual ruthenium oxide layer.According to other embodiments, other mixtures of the layer 46 arepossible, include regions of a solid solution of ruthenium metal andruthenium oxide.

If desired, resistive switching metal oxide structures may be formed byheating multiple layers of metal-containing material in the presence ofone or more layers of oxide. For example, multiple metal-containinglayers such as layer 40 may be deposited beneath the oxide layer 44.With this type of approach, two, three, or more than three metal oxidelayers and a metal overlayer (e.g., an electrode) can be formed.Following heat treatment, metal from each of the metal-containing layers40 may be oxidized by oxygen from the adjoining oxide layer 44. Thisapproach can be used to form structures with metal oxide layers formedfrom the same materials or different materials.

If desired, layer 40 and/or layer 44 may be formed from multiplesublayers, as shown by illustrative sublayers 44 in FIG. 10. Insituations in which the oxide layer 44 is formed from multiple oxidesublayers, the oxygen from each sublayer may be used in oxidizing themetal of metal-containing layer(s) 40 (including any sublayers of theselayers). In situations in which layer 44 is formed from multiplemetal-containing sublayers, the oxygen from the oxide may be used inoxidizing each of the metal-containing sublayers. Some or all of theoxygen in each oxide sublayer may be consumed and some or all of themetal in each metal-containing sublayer may be consumed during heattreatment.

An illustrative arrangement in which layer 44 is made up of multiplesublayers is shown in FIG. 10. In this example, layer 44 includes twosublayers. Layer 44A is formed from hafnium. Layer 44B is formed fromtitanium nitride. The use of two sublayers (layers 44A and 44B) and theuse of hafnium and titanium nitride as the materials in thesemetal-containing layers is merely illustrative. Layer 44 may, ingeneral, include any suitable number of sublayers and such sublayers maybe formed from any suitable metal-containing materials.

As shown in the upper portion of FIG. 10, an oxide layer 46 such as ametal oxide layer may be formed on top of the sublayers in layer 44.This structure may be heated in a furnace or rapid thermal annealingtool. During heating, oxygen from oxide layer 40 may oxidize the hafniumin layer 44A and may oxidize the titanium in titanium nitride layer 44B.

Depending on process conditions such as the amount of heat applied andthe length of heating, the oxide layer 46 may partially or fullydisassociate and the metal containing layers 44A and 44B may partiallyor fully oxidize. In the example shown in the lower half of FIG. 10,oxide layer 46 has completely disassociated, so only metal M remains inlayer 46 following heating. Hafnium layer 44A has oxidized to formhafnium oxide layer 48A. Some of the titanium in titanium nitride layer44B has oxidized to form titanium oxide layer 48B. A residual layer 40of titanium nitride has not been oxidized and remains under layer 48B.

As this example demonstrates, during heat treatment, oxygen from oxidelayer 46 may oxidize both sublayers 44A and 44B in metal-containinglayer to produce corresponding metal oxide sublayers 48A and 48B inresistive switching metal oxide layer 48. If desired, this approach maybe used with oxide layers 40 that include multiple sublayers and/or withstacks that include multiple alternating layers 40 and 44 (with orwithout sublayers in each oxide layer and/or metal-containing layer).

An advantage of the structures formed in FIGS. 9 and 10 is that the heattreatment results in the in-situ formation of a multilayer resistiveswitching structure (i.e., the structures shown in FIGS. 9 and 10) withpotentially smooth interfaces between layers. The structure of FIG. 9,for example, may be used as a nonvolatile resistive switching memoryelement, because it contains a resistive switching layer 48 that islocated between two conductive layers (e.g., metal layer 46 andmetal-containing layer 40 and/or conductive layer(s) in layer 42). Theinterfaces formed between layers 46 and 48 and between layers 48 and 40may be smooth and may exhibit good adhesion properties, because theyavoid the rough oxide/metal interfaces and poor adhesion that are oftenencountered when depositing metal oxides on metal layers and vice versa.

Illustrative steps involved in forming resistive switching metal oxidestructures by heating layers of oxide and metal-containing material areshown in FIG. 11.

At step 50, one or more lower-level layers of material such as asubstrate material (e.g., silicon, glass, etc.), conductors such asconductors 20, 24, 16, and 18 of FIGS. 2A and 2B, etc. may be formed.

At step 52, a metal-containing layer 40 may be formed on top of thelower-layer layers. The metal-containing layer 40 may include anysuitable metal or metals (e.g., a metal, a metal alloy, etc.), andoptional additional materials (e.g., nitrogen). Examples of metals thatmay be included in layer 40 include, Ti, Al, Hf, Ta, Zr, V, Nb, Cr, Mo,W, Mn, and Mo (as examples). Layer 40 may include a single layer ofmaterial or multiple sublayers of materials.

During step 52, metal-containing layer 40 may be formed on layer 42using any suitable technique such as physical vapor deposition (e.g.,sputter deposition, evaporation), chemical vapor deposition, atomiclayer deposition, and electrochemical deposition (e.g., electrolessdeposition or electroplating).

At step 54, oxide layer 44 may be deposited on metal-containing layer40. Oxide layer 44 may be formed as a single layer of material or mayinclude multiple sublayers of materials. As described in connection withFIG. 8, oxide layer 44 may include any oxygen-containing materials thatcan serve as suitable sources of oxygen for a resistive switching layersuch as ruthenium oxide, iridium oxide, etc. Suitable techniques forforming oxide layer 44 on metal-containing layer 40 include physicalvapor deposition (e.g., sputter deposition, evaporation), chemical vapordeposition, atomic layer deposition, and electrochemical deposition(e.g., electroless deposition or electroplating).

If desired, steps such as steps 52 and 54 may be repeated one or moretimes to form a multilayer structure (e.g., a structure in which thelayer 40 contains multiple metal layers and/or in which the layer 44contains multiple metal oxide layers).

At step 56, heat may be applied to the layers by placing the structuresin a furnace or using a rapid thermal annealing (RTA) tool. Duringheating, one or more resistive switching metal oxide switching layersmay be formed. In particular, some or all of the oxygen from the oxidelayer(s) 44 may oxidize some or all of the metal in metal-containinglayer 40 (including the metal in any sublayers within layer 40). Theresulting structures may include a residual layer formed from oxidelayer 44 such as residual layer 46. Residual layer 46 may, for example,include residual metal and/or metal oxide from layer 44. In situationsin which the oxygen from oxide layer 44 is fully consumed, layer 46 mayinclude only metal (or a metal alloy). In situations in which the oxygenfrom oxide layer 44 is partially consumed, layer 46 may include residual(unconsumed) oxide such as a layer of unconsumed metal oxide (as anexample). Similarly, metal-containing layer 40 may or may not be fullyconsumed during processing. In situations in which metal-containinglayer 40 is only partially consumed, a portion of the metal-containinglayer may remain in the resistive switching memory element structure, asshown in FIG. 9.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed:
 1. A method for fabricating a resistive switching memory element, comprising: forming a first layer that contains a first metal; forming a second layer that contains a second metal and oxygen; forming a resistive switching layer by heating the first and second layers so that the oxygen from the second layer oxidizes the first metal; and forming a metal layer from the second metal, wherein forming the metal layer comprises forming a second electrode.
 2. The method defined in claim 1 wherein forming the second layer comprises forming a metal oxide.
 3. The method defined in claim 1 wherein forming the second layer comprises forming a binary metal oxide.
 4. The method defined in claim 1 wherein forming the first layer comprises forming a metal nitride and wherein forming the second layer comprises forming a metal oxide.
 5. The method defined in claim 1 wherein forming the first layer comprises forming a metal nitride, wherein forming the second layer comprises forming a metal oxide, and wherein forming the resistive switching layer comprises heating the first and second layers sufficiently that the oxygen from the second layer oxidizes the first metal of the first layer to form a metal oxide that exhibits resistive switching.
 6. The method defined in claim 5 wherein heating the first and second layers sufficiently comprises heating the first and second layers using rapid thermal annealing.
 7. The method defined in claim 6 wherein heating the first and second layers to generate a layer adjacent to the resistive switching metal oxide formed of only the metal of the second layer.
 8. The method defined in claim 6 wherein heating the first and second layers using rapid thermal annealing comprises heating the first and second layers to generate a layer adjacent to the resistive switching metal oxide formed of the second metal of the second layer while leaving a portion of the first layer unoxidized.
 9. The method defined in claim 1 further comprising: forming at least one additional layer that contains metal between the first layer and the second layer; and while heating the first and second layers, heating the additional layer so that the oxygen from the second layer oxidizes the metal from the additional layer.
 10. The method defined in claim 1 wherein the first layer contains multiple metals and wherein forming the resistive switching metal oxide layer comprises oxidizing the multiple metals with the oxygen from the second layer by heating the first and second layers. 